The present invention relates to electrochemical conversion cells, commonly referred to as fuel cells, which produce electrical energy by processing first and second reactants. For example, electrical energy can be generated in a fuel cell through the reduction of an oxygen-containing gas and the oxidation of a hydrogenous gas. By way of illustration and not limitation, a typical cell comprises a membrane electrode assembly (MEA) positioned between a pair of flowfields accommodating respective ones of the reactants. More specifically, a cathode flowfield plate and an anode flowfield plate can be positioned on opposite sides of the MEA. The voltage provided by a single cell unit is typically too small for useful application so it is common to arrange a plurality of cells in a conductively coupled “stack” to increase the electrical output of the electrochemical conversion assembly.
The membrane electrode assembly typically comprises a proton exchange membrane separating an anode layer and a cathode layer of the MEA. The MEA is typically characterized by enhanced proton conductivity under wet conditions. For the purpose of describing the context of the present invention, it is noted that the general configuration and operation of fuel cells and fuel cell stacks is beyond the scope of the present invention. Rather, the present invention is methods of controlling the water content in the electrochemical conversion cell, specifically by regulating the relative humidity of an end cell via a heater.
During operation, a fuel cell stack is susceptible to loss of heat to the environment (e.g., conductive heat loss through attached hardware), particularly at the ends of the fuel cell stack. This loss of heat results in the temperature of the fuel cell stack being non-uniform along its length, with the end cells of the fuel cell stack comprising lower temperatures than the rest of the cells. Due to the temperature drop, water passing through the fuel cell may condense in the relatively cooler cells at the end of the fuel cell stack.
Condensation of water within the fuel cells at the end of the fuel cell stack is problematic since water can block the flow channels and flood the fuel cell. Flooding decreases voltage by not allowing reactants to reach the reaction sites and overall performance of the fuel cell stack decreases. In addition, flooding may also result in dehydration in other areas of the fuel cell stack. As a result, there is a continuing demand to control the water content of a fuel cell stack and the individual fuel cells making up the fuel cell stack.
Regarding the general configuration and operation of fuel cells and fuel cell stacks, applicants refer to the vast collection of teachings covering the manner in which fuel cell “stacks” and the various components of the stack are configured. For example, a plurality of U.S. patents and published applications relate directly to fuel cell configurations and corresponding methods of operation. More specifically, FIGS. 1 and 2 of U.S. Patent Application Pub. No. 2005/0058864 (now U.S. Pat. No. 6,974,648) and the accompanying text present a detailed illustration of the components of one type of fuel cell stack and this particular subject matter is expressly incorporated herein by reference.